Subject to change without notice
20
Testing Resistors
If the test object has a linear ohmic resistance, both deflecting
voltages are in the same phase. The test pattern expected
from a resistor is therefore a sloping straight line. The angle
of slope is determined by the resistance of the resistor under
test. With high values of resistance, the slope will tend
towards the horizontal axis, and with low values, the slope
will move towards the vertical axis.
Values of resistance from 20
20
20
20
20
Ω
to 4.7k
4.7k
4.7k
4.7k
4.7k
Ω
can be approximately
evaluated. The determination of actual values will come with
experience, or by direct comparison with a component of a
known value.
Testing Capacitors and Inductors
Capacitors and inductors cause a phase difference between
current and voltage, and therefore between the X and Y
deflection, giving an ellipse-shaped display. The position and
opening width of the ellipse will vary according to the
impedance value (at 50Hz) of the component under test.
•
A horizontal ellipse indicates a high impedance or a
relatively small capacitance or a relatively high inductance.
•
A vertical ellipse indicates a small impedance or a relatively
large capacitance or a relatively small inductance.
•
A sloping ellipse means that the component has a
considerable ohmic resistance in addition to its reactance.
The values of capacitance of normal or electrolytic capacitors
from 0.1
0.1
0.1
0.1
0.1
µ
F
F
F
F
F to 1000
1000
1000
1000
1000
µ
F
F
F
F
F can be displayed and approximate
values obtained. More precise measurement can be obtained
in a smaller range by comparing the capacitor under test with
a capacitor of known value. Inductive components (coils,
transformers) can also be tested. The determination of the
value of inductance needs some experience, because
inductors have usually a higher ohmic series resistance.
However, the impedance value (at 50Hz) of an inductor in
the range from 20
Ω
to 4.7k
Ω
can easily be obtained or
compared.
Testing Semiconductors
Most semiconductor devices, such as diodes, Z-diodes,
transistors, FETs can be tested. The test pattern displays
vary according to the component type as shown in the figures
below.
The main characteristic displayed during semiconductor
testing is the voltage dependent knee caused by the junction
changing from the conducting state to the non conducting
state. It should be noted that both the forward and the reverse
characteristic are displayed simultaneously. This is a two-
terminal test, therefore testing of transistor amplification is
not possible, but testing of a single junction is easily and
quickly possible. Since the test voltage applied is only very
low, all sections of most semiconductors can be tested
without damage. However, checking the breakdown or
reverse voltage of high voltage semiconductors is not
possible. More important is testing components for open or
short-circuit, which from experience is most frequently
needed.
Testing Diodes
Diodes normally show at least their knee in the forward
characteristic. This is not valid for some high voltage diode
types, because they contain a series connection of several
diodes. Possibly only a small portion of the knee is visible. Z-
diodes always show their forward knee and, up to approx.
7V, their Z-breakdown, forms a second knee in the opposite
direction. A Z-breakdown voltage of more than 6.8V can not
be displayed.
Type:
Normal Diode
High Voltage Diode
Z-Diode 6.8V
Terminals:
Cathode-Anode
Cathode-Anode
Cathode-Anode
Connections:
(CT-GD)
(CT-GD)
(CT-GD)
The polarity of an unknown diode can be identified by
comparison with a known diode.
Testing Transistors
Three different tests can be made to transistors: base-emitter,
base-collector and emitter-collector. The resulting test
patterns are shown below.
The basic equivalent circuit of a transistor is a Z-diode between
base and emitter and a normal diode with reverse polarity
between base and collector in series connection. There are
three different test patterns:
N-P-N Transistor
Terminals:
b-e
b-c
e-c
Connections: (CT-GD)
(CT-GD)
(CT-GD)
P-N-P Transistor
Terminals:
b-e
b-c
e-c
Connections: (CT-GD)
(CT-GD)
(CT-GD)
For a transistor the figures b-e and b-c are important. The
figure e-c can vary; but a vertcal line only shows short circuit
condition. These transistor test patterns are valid in most
cases, but there are exceptions to the rule (e.g. Darlington,
FETs). With the
COMP. TESTER
, the distinction between a
P-N-P to an N-P-N transistor is discenible. In case of doubt,
comparison with a known type is helpful. It should be noted
that the same socket connection (
COMP. TESTER
or ground)
for the same terminal is then absolutely necessary. A
connection inversion effects a rotation of the test pattern by
180 degrees round about the center point of the scope
graticule.
In-Circuit Tests
Caution! During in-circuit tests make sure the circuit
is dead. No power from mains/line or battery and no
signal inputs are permitted. Remove all ground
connections including Safety Earth (pull out power
plug from outlet). Remove all measuring cables
including probes between oscilloscope and circuit
under test. Otherwise both COMP. TESTER leads are
not isolated against the circuit under test.
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